Where can I get a users Manual for Casio Keyboard CT-648?

No you can't as our PROPER Expert Cindy says.

Casio Ct-636 does not support any sustain pedals.Amazon.comhttps://www.amazon.com > ask > questions
A: Casio Ct-636 does not support any sustain pedals.

The other 'Contributor' probably saw this

CT-S1000V - SupportCasio https://support.casio.com > manualPDF

This Digital Keyboard does not come with a pedal unit. • You can use the PEDAL1 jack to connect an optional (SP-3, SP-20) sustain pedal. You can use the.

try this... Casio Casiotone CT 360 Manuals ..or this... Casio CT 360 keyboard manual Google Search

https://dl.dropboxusercontent.com/u/7502291/keyboard-manuals/casio-ct-650_manual.pdf

http://dl.dropboxusercontent.com/u/7502291/keyboard-manuals/casio-ct-360_manual.pdf

There is no such model.
If you are referring to CT-615, you can purchase a PDF copy of manual from ebay

http://www.ebay.com.au/itm/Casio-CT615-Keyboard-Users-Owners-Manual-CTK-615-/200932122034

Some Casio Keyboards use 9V DC (center negative) others 12V DC (center negative).
If check what voltage your Keyboard use. Generic AC adapters are usually center positive. So you have to invert the wire with a new DC plug to make it center negative,
Easiest is to buy a Casio ac adapter made for your keyboard from a Casio retailer.

Here is the link to the user manual

http://support.casio.com/en/manual/008/CTK691_EN.pdf

http://generror.wordpress.com/2010/07/04/casio-keyboard-manuals/

The only places I found for the manual were pay sites.   Here is one for you to choose from.   They both were the same price.
http://www.user-manuals.com/advanced_search_result.php?keywords=casio+ct-420+manual&forward=plist

Here is the site to download the manual:http://safemanuals.com/user-guide-instructions-owner-manual/CASIO/CT-395-_E